1. Field of the Invention
This invention relates to a method of removing a mobile sensor apparatus from the header of a tubular heat-exchanger and returning the apparatus to its original position. The invention also relates to a device for carrying out this method.
2. Summary of the Prior Art
It is necessary to inspect the water tubes of vertical heat-exchangers employed in nuclear-powered generating stations. It is, in practice, essential that the water constituting the primary fluid and contaminated by nuclear reactions should not pass into the secondary fluid which is employed to drive a turbo-alternator. This inspection is effected by means of an eddy current sensor which is passed through each of the tubes; a crack or a hole in the tube or even a simple reduction in the thickness of the latter then becomes apparent as a change in the output current of the sensor.
In practice, the sensor is carried by a mobile apparatus disposed in the lower header of the heat-exchanger into each of which these tubes open. This mobile apparatus is secured to the heat-exchanger structure with the aid of expansible mandrels which are introduced into it and which are secured within the tubes of the tube bundle, and carries a guide tube which lies opposite another tube of the bundle and into which the eddy current sensor is introduced. After having inspected one tube, the sensor is retracted from the tube, the apparatus is moved a distance equal to the spacing between two tubes and the sensor is introduced into a fresh tube. These various operations are controlled externally since the wall of the header having been in contact with the irradiated water itself emits dangerous radiation.
If a fault occurs in the sensor apparatus preventing its displacement, a repair man must then enter the header in order to overcome the fault. To do this, it is necessary to provide protective clothing and make use of a face mask, and, despite this, no one can remain in the header except for a very short period, of the order of three minutes. This poses a serious problem, since it may be either that the repair requires more than three minutes, or that there are no repair men authorized to enter the chamber.
An object of the present invention is to provide a method and an apparatus by which it is possible to retrieve the mobile sensing apparatus from the lower header of a tubular heat-exchanger without having to enter that header.